In ion cyclotron resonance (ICR) mass spectrometers, the mass-to-charge ratio of ions is measured by the frequency of their cyclotron oscillations in a homogenous magnetic field of high field strength. The magnetic field is usually produced by superconducting magnet coils, offering usable diameters of homogenous field from four to eight inches. Modern ICR mass spectrometers operate at magnetic fields of 7 to 12 Tesla flux density.
The ion cyclotron frequency is measured in ICR traps mounted inside the magnetic field. The ICR trap consists of at least four lengthy outer electrodes parallel to the magnetic field lines. At least two of the electrodes are used to synchronously accelerate the cyclotron motion by application of an oscillating electric field in resonance with the cyclotron frequency of the ions; and at least two electrodes are used as probes for the cyclotron motion of the ions by measuring their image currents induced in those electrodes. Ion acceleration and ion detection are performed in subsequent phases. Because the mass-to-charge ratio of the ions is unknown before measurement, the resonant acceleration of the ions (their “excitation”) is performed by a mixture of excitation frequencies. The mixture of frequencies can be produced either as a mixture in time (a “chirp pulse”) or as a synchronous mixture of frequencies (a “sync pulse”).
The image currents induced by the cyclotron motion of the ions at the probe electrodes (“detection electrodes”) are amplified, digitized, and investigated by Fourier transformations with respect to inherent cyclotron frequencies indicating the presence of corresponding ions. The raw signals obtained at the probe electrodes are called the “time domain signals,” and the Fourier transformed values are denominated “frequency domain signals,” containing peaks for the frequencies of ions. From these frequencies, the mass-to-charge ratios of the ions can be calculated with high accuracy and precision. Because of the application of Fourier transform mathematics, these ICR mass spectrometers are called “Fourier transform mass spectrometers” (FTMS). At present, Fourier transform mass spectrometry is the most precise method for the measurement of ion masses.
The electrodes of an ICR trap usually form either a trap with a square or a circular cross section. Other shapes are occasionally used, too. Four plane electrodes with small electrically insulating distances form a square trap. A cylindrical trap is formed by four (or more) lengthy electrode segments forming a cylinder (see FIG. 2). Cylindric ICR traps are frequently used because they better make use of the full volume of the magnetic field.
Since the motion of ions parallel to magnetic field lines do not induce a Lorentz force, ions with a velocity component parallel to the magnetic field can easily escape the detection region. To prevent the loss of ions by this effect, the ICR traps have to be equipped at both ends by a pair of electrodes which are connected to an ion-repelling DC voltage to keep the ions inside the trap. These electrodes are usually called “trapping electrodes.” Very different forms of trapping electrodes are known, but in the most simple case, the trapping electrodes are just plane electrodes (“trapping plates”) with central holes. The central holes serve to introduce ions into the ICR trap. The repelling voltages at these trapping electrodes form a potential well along the axis of the ICR trap, only weakly dependent on the precise shape of the trapping electrodes. The potential well along the axis has the form of a parabola, with a minimum exactly in the center of the trap, if the trapping potentials applied to both the trapping electrodes are equal. Ions located in the axis of the trap, having some kinetic energy in the direction of the axis (left over from the process of entering the trap) oscillate between the trapping electrodes, the width of the “trapping oscillation” depending on their kinetic energy.
Outside the axis of the ICR trap, the electric field formed by the trapping electrodes is more complicated and necessarily shows electrical field components in radial direction, as the detection and excitation electrodes usually have no DC voltages. These field components induce another type of movement, the “magnetron” motion. The magnetron motion around the axis of the ICR trap is much slower compared to the cyclotron motion and leads the center of the fast cyclotron oscillation around the ICR trap axis on a magnetron circle, resulting in a cycloidal movement.
The superposition of the magnetron oscillation and the cyclotron oscillation is, in principle, a nasty effect introducing a frequency shift, diminishing the usable volume for the cyclotron movement, and reducing the mass resolution. The measured frequency ωm (“reduced cyclotron frequency”) of the ions amounts to
            ω      m        =                            ω          c                2            +                                                  ω              c              2                        4                    -                                    ω              t              2                        2                                ,whereby ωc is the cyclotron frequency, and ωt is the frequency of the trapping oscillation, the latter describing the influence of the magnetron motion. An ICR trap without magnetron oscillation would be of big advantage, because the cyclotron frequency ωc could be measured directly, and no corrections were needed.
The vacuum in the ICR trap should be as good as possible, because no collisions should occur during the measurement of the ion movements. Vacua in the range of 10−7 to 10−8 Pascal are usually applied. Usually, measurements of the time domain signals for the determination of the cyclotron frequency have durations from some hundred milliseconds to several seconds.